Abstract
Streptococcus suis (S. suis) and Actinobacillus pleuropneumoniae (A. pleuropneumoniae) are primary swine pathogens that have been frequently co-isolated from pigs suffering from severe respiratory disease. The purpose of this study was to investigate the biological impacts of the interactions between S. suis and A. pleuropneumoniae. A single- and dual-species culture model was established in vitro via S. suis HA9801 (serotype 2) and A. pleuropneumoniae CVCC265 (serotype 1). The single or mixed biofilms were imaged by confocal laser scanning microscopy. The biomass and viable cells in biofilms were quantified by crystal violet staining and determination of colony-forming units. The antibiotic susceptibility was determined by a microdilution broth method. The differences in gene transcription in pure- or mixed-species biofilms of S. suis and A. pleuropneumoniae was evaluated by quantitative PCR. S. suis and A. pleuropneumoniae formed two-species biofilms when co-cultured in vitro. When co-cultured with S. suis, biofilm formation by A. pleuropneumoniae was significantly increased with the absence of NAD that is necessary for the growth of A. pleuropneumoniae. Moreover, compared with monocultures, the antibiotic resistance of S. suis and A. pleuropneumoniae was both enhanced in the co-culture model. When grown in dual-species biofilms, for A. pleuropneumoniae, genes associated with virulence factors, including exotoxins and adhesins, were significantly upregulated. For S. suis, virulence factor-related genes cps2, gdh, mrp, and sly were highly induced. These results suggest that the interspecies interactions between S. suis and A. pleuropneumoniae may be cooperative under specific conditions and may play an important role in the disease progression and persistent infection.
Highlights
Respiratory diseases constitute the most important health issues affecting the swine industry worldwide and are often referred to as the porcine respiratory disease complex (PRDC) (Qiao et al, 2011)
The Competitive Index (CI) was significantly higher than the Relative Increase Ratio (RIR) between 8 and 24 h (p < 0.05), suggesting a noticeable negative effect of A. pleuropneumoniae on S. suis growth
Pathogens associated with PRDC include swine influenza virus (SIV), porcine circovirus type 2 (PCV2), porcine reproductive and respiratory syndrome virus (PRRSV), S. suis, A. pleuropneumoniae, Mycoplasma hyopneumoniae, and Haemophilus parasuis
Summary
Respiratory diseases constitute the most important health issues affecting the swine industry worldwide and are often referred to as the porcine respiratory disease complex (PRDC) (Qiao et al, 2011). PRDC is multifactorial in nature and is triggered by mixed infections involving primary and secondary pathogens (Cheong et al, 2017). Streptococcus suis (S. suis) and Actinobacillus pleuropneumoniae (A. pleuropneumoniae) are two important respiratory pathogens frequently detected in the PRDC (Opriessnig et al, 2011). There have been many cases of mixed infections associated with the presence of S. suis and A. pleuropneumoniae in China. It is more than likely that the different bacteria or viruses coexisting in PRDC interact and exacerbate the pathogenesis of the disease. Few research groups investigated the interactions between S. suis and A. pleuropneumoniae, except for a study reporting that these two bacterial species form polymicrobial biofilms in vitro (Ramirez-Castillo et al, 2018)
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